1. Field of the Invention
The present invention relates to a structure of a liquid crystal display where a driver circuit is provided for pixels as in a Thin Film Transistor (TFT) liquid crystal display, and where a relatively small number of rows are driven in a time division manner.
2. Description of the Related Art
Driving method for a liquid crystal display is classified into a method for passive-matrix or a method for active matrix. A passive matrix liquid crystal display device has been employed for a display targeted for still image display such as a PDA, because of its simple structure and cost effectiveness. On the other hand, an active matrix liquid crystal display device, which can realize a high contrast ratio display as well as a multi-color display with ease, has been widely used for a personal computer, workstation, or the like.
The passive matrix liquid crystal display device is constructed such that a liquid crystal layer is held between a group of row electrodes and a group of column electrodes, and pixels are arranged in matrix. A pixel unit composed of plural electrodes is driven in a time division manner. Example of driving method includes voltage averaging method, smart addressing (SA) method, and multi-line addressing (MLA) method.
The active matrix liquid crystal device has such a structure that active elements turn individual pixels ON/OFF to display an image, and is represented by a TFT display which uses transistors as active elements. As a driving method for a TFT liquid crystal display, the following three methods have been mainly put into practical use; a dot inversion driving method, a line inversion driving method, and a frame inversion driving method. In these methods polarity of an AC signal is inverted on every pixel, every scanning line, and every frame, respectively. Among those, the dot inversion driving method offers the highest display quality. The line inversion driving method offers the second highest display quality and somewhat causes crosstalk. The frame inversion driving method causes large crosstalk in a vertical direction, leading to a non-uniform image with a brightness gradient in a vertical direction on a screen. On the other hand the frame inversion driving method consumes the lowest power. The line inversion driving method consumes power about three times the power in the frame inversion driving method. The dot inversion driving method consumes power about six times the power in the frame inversion driving method. If an effective AC inversion period is shortened by switching the AC signal applied to a liquid crystal at every pixel or at every scanning line in these driving methods which have lower display qualities, the polarity of the signal applied to the liquid crystal changes area by area, thereby flickering of the screen is reduced (see JP 05-029916 B, for example).
Amorphous silicon or poly-crystalline silicon has been used widely as a substrate for a TFT liquid crystal display. Hereinafter a-Si TFT is used for a TFT formed on an amorphous silicon substrate and p-Si TFT for a TFT formed on a poly-crystalline silicon substrate. In recent years, however, a liquid crystal display device using crystalline silicon as a TFT substrate (hereinafter c-Si TFT) is coming into the market.
Typical manufacturing methods for the c-Si TFT are:    (1) Forming an image display portion where transistors are arrayed directly on a c-Si wafer and using the resultant as a display driving substrate as it is;    (2) Forming a display driving substrate by forming an image display portion where transistors are arrayed directly on the c-Si wafer and then bonding the surface having the circuit formed thereon onto a glass substrate, followed by grinding/polishing the rear surface to connect a pixel electrode through wiring; and    (3) Forming a display driving substrate by forming a transistor circuit element on a c-Si wafer, grinding/polishing the wafer into a thin film, separating the transistor circuit elements from one another by anisotropic etching, arranging the separated transistor circuit element in a hole corresponding to the image display portion on the substrate in a liquid, and then forming an electrode. In addition to the above methods, for example, laser annealing of amorphous silicon is currently under study (see “Information Display: Vol. 15, No. 11, November 1999” for example).
In time-division driving as in the passive matrix driving, as the number of row electrodes increases, an ON/OFF ratio of an effective voltage applied to the liquid crystal reduces, resulting in a low contrast. Accordingly, there is a limitation on the number of row electrodes to which a voltage can be applied in practical use. Hence, disadvantageously, the passive matrix is not suitable for a panel having a larger size. Meanwhile, in the case of the active matrix driving, an AC signal is used for driving, resulting in flickering on the screen. If the dot inversion driving is adopted to reduce the flicker, the power consumption disadvantageously increases.
As regards a substrate on which a TFT is integrated, a conventional liquid crystal display device using a-Si TFT or p-Si TFT can produce a low-cost, large-area liquid crystal display device. However, because of a low mobility of the transistor, there is a disadvantage in that the device neither reduces an element size nor allows a high-speed operation. According to the method using the c-Si wafer itself as an image display region, electrons/holes leak in the wafer thickness direction or a floating capacitor is formed in the same direction, for example. Thus, with the method, the transistor is incapable of operating at a high speed in comparison with other methods using single-crystal silicon. Also, the method requires the c-Si wafer having the same area as that of the display panel and thus is disadvantageous in that it is not suitable for the panel with a larger size or lower cost.
Further, in a method where the transistor circuit elements are formed on the c-Si wafer, the wafer is ground/polished into a thin film, and the transistor circuit elements are separated by anisotropic etching and then arranged on the substrate in a liquid, it is unnecessary to form a portion other than the transistor circuit such as a wiring in a c-Si process. As a result, a liquid crystal display device using a c-Si TFT can be manufactured at a relatively low cost. However, there is a disadvantage in that the separated transistor circuit element has a large size, narrowing a transparent electrode formation region and reducing an opening ratio. Also, in the case where the elements are arranged in the pixels in a one-to-one correspondence, the c-Si TFT is higher in cost than the a-Si TFT.